Monday, 29 June 2009

Learning how to tell time with cyanobacteria

Most living things can innately tell the time. If you take a person and put them into continuous darkness for months by themselves they will follow a sleep-wake cycle of about 24-25 hours (as well as going a little mad). Circadian rhythms allow life to predict the raising and setting of the sun to allow us to change our gene expression metabolism and behaviour with the changing of the day. Predicting changes in temperatures and light levels are very important. Circadian clocks have three important features. First of all they are free-running at about 24 hours (in constant conditions like light or darkness they will keep going at ~24 hours). Secondly they are temperature compasatory meaning that changes in temperature do not greatly affect the clock. Finally they must be able to be reset by environmental inputs. The changes of the seasons mean days get longer or shorter so to accurately predict the raising of the sun your clock must be reset. Plus for those who fly great distances we must reset according to the new environment and sadly suffer from jet lag while we are slowly adjusting (Johnson et al., 2008).

These clocks have evolved independently multiple times. In animals, plants, some fungi and recently discovered in cyanobacteria (Johnson et al., 2008). It was once thought because prokaryotes divide so raplidly that a 24 hour rhythm would be useless to them but that is not the case (Johnson et al., 1996). A great deal has been learnt about rhythms in bacteria but there are still many unanswered questions. The fact circadian clocks have evolved multiple times shows it offers a great advantage to those that posses one but was no present in the last common ancestor of life. Changing behaviour to only occur at a specific time of day is a great advantage to organisms, plants to photosynthesize and animals to look for food at the right times (Bell-Pedersen et al., 2005).

Cyanobacteria also rely on the sun for energy so predicting when the sun will rise allows them to start synthesising photosystem proteins before the sun raises while not producing them all night or waiting for the sun to raise. It has been shown when you make a mixed culture of bacteria with a mutant clock (no rhythmic changes in gene expression) and wild-type then wild-type will soon outcompete the mutants. Those that have a different length period (eg 16 hours rather than 24 hours) will always lose to wild-type in 24 hour day conditions but wild-type will lose in conditions that match the internal clock of these mutants (Yan et al., 1998). Interestingly in cyanobacteria the whole genome shows rhythmic expression! (Liu et al., 1995) This is in contrast to most organisms that control 15%-35% of their genome in a circadian dependant manner (McDonald and Rosbach, 2001; Correa et al., 2003; Michael and McClung, 2003). This is controlled by the supercoiled state of the whole genome! (Smith and Williams, 2006) I eagerly await the paper that explains how this achieved.

What is rather fascinating about this system is the oscillator. In most organisms gene expression is negatively regulated by its product in a way it cycles its levels over 24 hours and controls output (Bell-Pedersen et al., 2005). In cyanobacteria, this is not the case. Rhythmic expression of clock proteins occur but they do not repress there own synthesis. Also an in vitro oscillator can be setup. In the test tube the clock protein KiaC will cycle through different phosphorylation states if with KaiA, KaiB and ATP. In the organism this forms the post-translational oscillator (Nakajima et al., 2005). This is temperature compensatory and this is built into the ATPase activity of KaiC (Terauchi et al., 2007). It has been shown even when locked in one phosphorylation state you get a weak rhythm so a transcriptional-translational feedback loop as in eukaryotes is needed for a robust system (Kitayama et al., 2008) posing the question of whether this occurs in eukaryotes.

And to finish off I will add one final oddity about the circadian rhythms of cyanobacteria. Instead of using a photoreceptor to sense light inputs like other organisms they read the state of the metabolism. When light is high photosynthesis is occurring and the redox state of the cell changes and is seen as an input to reset the clock (Ivleva et al., 2005).

Smith, R.M. and Williams, S.B. (2006) Circadian rhythms in gene transcription imparted by chromosome compaction in the cyanobacterium Synechococcus elongatus. Proceedings of the National Academy of Sciences of the United States of America, 103, 8564-8569.

Terauchi, K., Kitayama, Y., Nishiwaki, T., Miwa, K., Murayama, Y., Oyama, T. and Kondo, T. (2007) ATPase activity of KaiC determines the basic timing for circadian clock of cyanobacteria. Proceedings of the National Academy of Sciences of the United States of America, 104, 16377-16381.

Yan, O.Y., Andersson, C.R., Kondo, T., Golden, S.S. and Johnson, C.H. (1998) Resonating circadian clocks enhance fitness in cyanobacteria. Proceedings of the National Academy of Sciences of the United States of America, 95, 8660-8664.

4 comments:

The fact that cyanobacteria regulate their entire genome is very interesting. Does that mean that they are pretty much dormant at night or just that they switch to activities other than photosynthesis? I hope someone figures out the supercoiling mechanism, it could be a really elegant system.

I agree that the control of the genome is the most interesting part. i was very disappointed to see how little work had been done on it compared to the core oscillator. from what i gathered some genes are on at different times of day, so a subset are night genes while i think the majority are day genes with some peaking at different times but how this is regulated is poorly understood (by me at least). what was interesting was when you mutate SasA (histadine kinase) you decrease amplitude of gene expression but the genome still condenses etc. a question i asked was if you mutate the response regulator RpaA that SasA feeds into does the genome still cycle through superoiled states? no one seems to have checked. the phenotype of rpaA mutants is stronger than sasA mutatnts so it seems plausible. i am waiting to see what happens.

Contributors

We are geneticists and biochemists, alumni of the University of York (2009), now doing PhDs at the Universities of Cambridge, Leeds, Oxford and Vermont. We aim to bring to your attention interesting science, whether it is making headlines or not, referencing the original peer-reviewed research as often as possible.